With regard to envelopes of metal vapor discharge lamps, envelopes made of translucent ceramic such as alumina ceramic have become increasingly common these days in place of conventional quartz glass. Translucent ceramic is more excellent in heat resistance than quartz glass and suitable for envelopes of high pressure discharge lamps, such as metal vapor discharge lamps, whose temperature becomes high when the lamps are on. For example, alumina ceramic has lower reactivity with light-emitting metals to be enclosed in an envelope than quartz glass, and it can thus be expected to prolong the life of metal vapor discharge lamps.
A typical envelope of a metal vapor discharge lamp comprises: a center bulb for defining a discharge space and a pair of side tubes being extended from both ends of the center bulb. The side tubes have outer diameters smaller than that of the center bulb. Current suppliers are extending through hollows of the side tubes respectively. The current supplier comprises a lead-in wire and an electrode fixed with a coil. The coil is disposed in the discharge space. The lead-in wire is fixed to the inside of the side tube by means of a sealant. The sealant hermetically seals open ends of the side tubes. As for the sealant used is glass frit or the like.
When a metal vapor discharge lamp is turned on in such a state as an electrode of the current supplier is oriented in the vertical direction, the light-emitting metal enclosed in the discharge space easily sinks into a gap between the lead-in wire and the side tube disposed on the lower side of the vertical direction. When the light-emitting metal sinks into the gap, an amount of the light-emitting metal to contribute to luminescence in the discharge space is reduced, resulting in insufficient vapor pressure and a larger variation in color temperature. It is often the case that, even if characteristics of a metal vapor discharge lamp are sufficient immediately after the lamp is turned on, the characteristics vary significantly several hundred or several thousands hours after the lamp is turned on. Although increasing the amount of the light-emitting metal can be considered as a means to prevent the abovementioned problem, such an increase may promote the reaction of the light-emitting metal with the electrode or ceramic, deteriorating the life characteristic of the lamp.
There has been proposed a metal vapor discharge lamp using an envelope where a center bulb has been bonded to side tubes by shrink-fitting. In this lamp regulated is a position of a coil to be disposed in the vicinity of an end of the electrode in the envelope. This regulation enables control of a temperature of the shrink-fitting portion to inhibit a light-emitting metal from sinking (Japanese Laid-Open Patent Publication No. 2000-340171). According to this proposal, the light-emitting metal in a liquid state can exist at the shrink-fitting portion of a low-temperature because the shrink fitting portion has a thickness larger than those of the center bulb and the side tubes. This makes it possible to reduce the amount of the light-emitting metal that sinks into the gap between the current supplier and each of the side tubes than in the conventional practice.
On the other hand, in a translucent ceramic envelope where a center bulb has been integrally molded with side tubes, the smallest curvature radius of an inner wall of a boundary portion between the center bulb and each of side tubes tends to become large. This is ascribable to a method of producing such an envelope. For this reason, in a metal vapor discharge lamp using the integrally molded envelope, a liquid light-emitting metal tends to flow down into the gap between the current supplier and each of the side tubes. Accordingly, it has been proposed that the smallest curvature radius of the inner wall of the boundary portion between the center bulb and each of the side tubes be controlled to a small value. The boundary portion so controlled is resistant to allowing the metal to flow thereon (Japanese Laid-Open Patent Publication No. 2002-164019).
However, in the case of shaping the boundary portion between the center bulb and each of the side tubes as described above, it becomes difficult to regulate the temperature of the boundary portion, raising a problem that favorable metal vapor pressure cannot be obtained. In order to obtain a metal vapor discharge lamp having a stable luminous characteristic, it is necessary to keep the boundary portion at such a temperature as favorable metal vapor pressure can be obtained as well as to control the shape of the boundary portion.